1. Technical Field
The present invention relates to a method of producing a printing ink-decorated packaging material of laminate type, which has, on its outside, applied decor of water-based printing.
2. Background Art
In the packaging industry, use is often made of liquid-tight, dimensionally stable packages of the single use disposable type for packing and transporting liquid foods.
Single use disposable packages of the type under consideration here are typically produced from a packaging material of laminate type comprising a rigid, but foldable core layer of paper which, at least on its one side, has an outer coating of plastic which serves as the decor carrier and which is preferably low density polyethylene (LDPE) having, applied on its outside, decor of water-based printing ink. Preferably, the core layer also has an outer coating of plastic, normally low density polyethylene (LDPE) on its other side as well, whereby the packaging material makes for the production of dimensionally stable, liquid-tight packages by means of simple, effective thermosealing.
In certain cases, in particular when the packaging material is to be employed for so-called aseptic packages, the packaging material is also provided with a material layer which serves as gas barrier and is disposed between the core layer and that one of the two outer plastic coatings which is intended to be turned to face inwards when the packaging material is reformed into packages. The material in this gas barrier layer may be a so-called barrier polymer, for example polyamide, or ethylene vinyl alcohol copolymer, but most generally consists of an aluminium foil (so-called Alifoil).
Nowadays, dimensionally stable, liquid-tight packages are most generally produced with the aid of modern, high-speed packing and filling machines of the type which, either from a web or from prefabricated sheet blanks of the packaging material, form, fill and seal the packages. From, for example a web, the packages are produced in that the web is first reformed into a tube by both longitudinal edges of the web being united with one another in an overlap joint seal. The tube is filled with the relevant contents, for example liquid food, and is divided into closed, filled packages by repeated transverse seals transversely of the longitudinal axis of the tube below the level of the contents in the tube. The packages are separated from one another by incisions or cuts in the transverse sealing zones and are given the desired geometric, normally parallelepipedic configuration, by a further forming and sealing operation for the formation of the finished packages.
So-called aseptic packages are produced fundamentally in the same manner, but with the crucial difference that both the packaging material and the relevant contents are sterilized prior to the filling phase, and that the filling phase takes place in a sterile (aseptic) filling atmosphere so as to avoid reinfection of the sterilized contents.
A sterilization of the packaging material often takes place with the aid of a chemical sterilizing agent, for example an aqueous solution of hydrogen peroxide, with which the packaging material is brought into contact for destroying unwanted, in particular pathogenic micro-organisms on at least those parts of the packaging material which come into contact, or risk coming into contact with the sterilized contents. However, the entire packaging material is often sterilized in that a web of the packaging material is led down into and through a bath of aqueous hydrogen peroxide which, for the above-mentioned purpose of sterilization, is allowed to act on the packaging material web prior to reforming of the web into aseptic packages as described above.
Printing inks for the described application in connection with packaging materials are often water-based and therefore require that the surface on which they are to be applied is sufficiently hydrophilic to achieve good adhesion between the water-based printing ink and the relevant printing surface.
A surface of polyethylene, e.g. LDPE, which is used as printing surface in the above described prior art packaging material is, however, hydrophobic (water repellent) and must therefore be modified in order to make for the desired good adhesion between printing ink and printing surface. For the purpose of promoting adhesion, the packaging material according to the prior art technique is therefore subjected to a surface treatment which changes its polarity by means of electric corona discharges, as a result of which the polyethylene surface obtains the desired hydrophilic character.
While it has thus previously been possible to produce a printing inkdecorated packaging material of the type described by way of introduction with good adhesion between the water-based printing ink and the outer polyethylene coating of the packaging material, it has surprisingly proved that the prior art packaging material, in particular for aseptic packages, may be improved considerably using only simple means.
By subjecting the printing ink-applied web of packaging material to a surface energy-reducing after treatment, as in the method according to the present invention, it is possible to produce a printing ink-decorated packaging material which not only withstands storage in a damp environment even for lengthy storage times, without the decor losing its clarity and freshness, but which also can be sterilized using aqueous hydrogen peroxide, as described above, with considerably lower consumption of hydrogen peroxide than has hitherto been possible using the prior art methods.
One explanation for the above described drawbacks inherent in the prior art printing ink-decorated packaging material, i.e. moisture sensitivity on storage and excessively high hydrogen peroxide consumption in connection with sterilization, may be that a corona treatment, at the same time as it realises modification (change of polarity) of the decor-carrying polyethylene coating of the packaging material, is sufficiently powerful that the polyethylene coating is xe2x80x9cbroken upxe2x80x9d when it is hit by the electric corona discharges. Within such xe2x80x9cbroken upxe2x80x9d regions the polyethylene coating wholly or partly lacks hydrophilic seats which are required for the waterbased printing ink to adhere with good adhesion, as a result of which the printing ink thus only partly adheres within these regions. The risk of interaction between printing ink and moisture or liquid and consequential deterioration in the quality of the applied decor has thus increased within the above mentioned regions.
The problem with xe2x80x9cbroken upxe2x80x9d material regions of the outer polyethylene coating of the packaging material becomes particularly serious when the packaging material is, for the purpose of sterilization, led through a bath of aqueous hydrogen peroxide, as in the production of aseptic packages. Apart from the fact that the printing ink comes into contact and interacts with the water in the hydrogen peroxide solution, hydrogen peroxide will be excessively attracted and absorbed by exposed hydrophilic seats in the polyethylene coating also in regions outside the applied printing ink decor, with excessive hydrogen peroxide consumption as a result.
By subjecting the corona-treated outer polyethylene coating of the packaging material to a surface energy-reducing after treatment by means of heat, the above described drawbacks in connection with the prior art packaging material are thus effectively obviated.
Preferably, the surface energy-reducing after treatment is carried out by means of hot press rollers or press cylinders between which the packaging material is led, whereby a physical levelling effect on the xe2x80x9cbroken upxe2x80x9d polyethylene coating is also achieved, which, together with the reduced surface energy, further contributes in counteracting the tendency of the corona-treated polyethylene coating to absorb liquid (water).